The usual industrial practice of catching fish or harvesting sea-food produce is to bring them on board of the fishing boat where processing of the catch is operated or not. Most of the time, fishing boats remain at sea for 5 days or more: therefore, it becomes necessary to process the catch as soon as possible and prevent it from turning bad. This is especially true for the catch obtaining during the early days at sea. A common practice is to store the catch in a refrigerated space mixed with an equivalent amount of crushed ice. When brought back to the port, the catch is either sold as fresh or more likely processed in various ways such as canning, smoking or freezing.
As soon as the catch is taken out of its living environment (sea water), degradation processes start. Chemical and bacteriological reactions are the major sources of fresh fish and sea-food degradations. Bacterial contamination has many origins. Part of it comes from ambient air, but handling, cleaning and storing operations are the major responsible factors for contamination of the product. When living in its natural environment, edible parts of fish or sea-food have an extremely low level of contamination (e.g., below 10 germs/g). However, it is not unusual to observe that level rising to 10.sup.4 or more germs/g after the product has been through every processing step. The higher the initial level of contamination, the faster the deterioration of the product will be and ultimately, part of the caught stock has to be destroyed because of its total loss of market value.
For a long period of time, it has been suggested to extend the shelf-life of fresh sea-food and fish by packaging these products under modified atmosphere in order to limit the microbial development on. It, has been suggested to use enriched or even pure CO.sub.2 atmospheres. It is known from U.S. Pat. Nos. 3,851,080 and 3,930,040 and French Patent 81/20784 to use a binary or ternary mixture of N.sub.2 and CO.sub.2, O.sub.2 and CO.sub.2 or O.sub.2, N.sub.2 and CO.sub.2, with partial pressures in the range of 50% N.sub.2 and 50% CO.sub.2 or 30% N.sub.2 and 70% CO.sub.2 or 20% O.sub.2 and 80% CO.sub.2. These compositions are appropriate and efficient for the extension of the shelf life of these products. CO.sub.2 has an inhibiting effect on a wide number of microbial germs and a good solubility within fats and moist products. Therefore, it has become a basic requirement in gas packaging techniques to use CO.sub.2 with a partial pressure as high as possible inside of the container. However, for moist products such as sea-food and fish, partial pressures of CO.sub.2 above 70% have the tendency to acidify the product. Some bleaching effect of the product due to elevated CO.sub.2 partial pressures may as well be observed. On the other hand, it has been shown that CO.sub.2 does not have an inhibition effect on the growth of strictly anaerobic germs such as Clostridium.
Fresh fish or sea-food can be naturally contaminated with spores of Clostridium botulinum. In strict anaerobic conditions, these spores are susceptible to germinate and produce a toxic protein which is excessively dangerous for mankind. Risk of development is particularly high when the storage temperature of the package exceeds 4.degree. C. This temperature has always been a difficult parameter to control all along the distribution chain of the product, and it is absolutely necessary to prevent the development of this germ in case of an accidental temperature rise. Introducing oxygen inside of the container or package is a recognized and accepted method to avoid Clostridium developments, and therefore an O.sub.2 partial pressure of at least 5% is usually recommended for the final formulation of the gas mix. Nitrogen is used as a complimentary gas providing space filling or dilution but it is not effective for controlling the microbial development.
Using the various gas combinations available today for sea-food or fish packaging extends its shelf-life from 2 days (air) to 8 days (gas). Exceptionally, products featuring a very low level of initial contamination and low fat content can have their shelf-life extended to 12-15 days. However, considering the length of the fishing campaign, the transportation time required for the distribution of the product and the storage time on distributor's shelves, a shelf-life of 15 days is a minimum requirement for an efficient marketing of any fresh sea-food or fish products.